Mechanical interface for proximity coupled electro-optic devices

ABSTRACT

The electrodes of a proximity coupled electro-optic device, such as a multi-gate light valve for an electro-optic line printer, are mechanically gapped from the electro-optic element to increase the uniformity of the electro-optic output of the device.

BACKGROUND OF THE INVENTION

This invention relates to electro-optic devices and, more particularly,to proximity coupled light valves for electro-optic line printers andthe like.

It has been shown that an electro-optic element having a plurality ofindividually addressable electrodes can be used as a multi-gate lightvalve for line printing. See, for example, a copending and commonlyassigned U.S. patent application of R. A. Sprague et al., which wasfiled June 21, 1979 under Ser. No. 040,607 on a "TIR Electro-OpticModulator with Individually Addressed Electrodes." Also see "Light GatesGive Data Recorder Improved Hardcopy Resolution." Electronic Design,July 19, 1979, pp. 31-32; "Polarizing Filters Plot Analog Waveforms,"Machine Design, Vol. 51, No. 17, July 26, 1979, p. 62; and "DataRecorder Eliminates Problem of Linearity," Design News, Feb. 4, 1980,pp. 56-57.

As is known, almost any optically transparent electro-optical materialcan be used as the electro-optic element of such a light valve. The mostpromising materials now appear to be LiNbO₃ and LiTaO₃, but there areother materials which qualify for consideration, including BSN, KDP,KD^(x) P, Ba₂ NaNb₅ O₁₅ and PLZT. In any event, the electrodes of such alight valve are intimately coupled to the electro-optic element and aredistributed in non-overlapping relationship widthwise of theelectro-optic element (i.e., orthogonally relative to its optical axis),typically on equidistantly separated centers so that there is agenerally uniform interelectrode gap spacing. A copending and commonlyassigned U.S. patent application of W. D. Turner, which was filed Sept.17, 1980 under Ser. No. 187,936 on "Proximity Coupled Electro-OpticDevices", shows that the electrodes of an electro-optic device, such asa multi-gate light valve, may be fabricated on a suitable substrate andpressed against or held very close to the electro-optic element toprovide what is referred to a "proximity coupling".

To perform line printing with a multi-gate light valve of the foregoingtype, a photosensitive recording medium, such as a xerographicphotoreceptor, is exposed in an image configuration as it advances in across line direction (i.e. a line pitch direction) relative to the lightvalve. More particularly, to carry out the exposure process, asheet-like collimated light beam is transmitted through theelectro-optic element of the light valve, either along its optical axisfor straight through transmission or at a slight angle relative to thataxis for total internal reflection. Furthermore, successive sets ofdigital bits or analog signal samples (hereinafter collectively referredto as "data samples"), which represent respective collections of pictureelements or pixels for successive lines of the image, are sequentiallyapplied to the electrodes. As a result, localized bulk or fringe fieldsare created within the electro-optic element in the immediate vicinityof any electrodes to which non-reference level data samples are applied.These fields, in turn, cause localized variations in the refractiveindex of the electro-optic element within an interaction region (i.e., alight beam illuminated region of the electro-element which is subject tobeing penetrated by the electric fields). Thus, the phase front orpolarization of the light beam is modulated (hereinafter genericallyreferred to as "p-modulation" of the light beam) in accordance with thedata samples applied to the electrodes as the light beam passes throughthe interaction region. Schlieren readout optics may be usedto convert aphase front modulated light beam into a light beam having acorrespondingly modulated intensity profile. For example, the phasefront modulated light beam may be imaged onto the recording medium bycentral dark field or central bright field imaging optics.Alternatively, if the input light beam is polarized, a polarizationmodulation to intensity modulation conversion process may be performedby passing the polarization modulated output beam through a polarizationanalyizer. In more generic terms, the p-modulation of the light beam isconverted into a correspondingly modulated intensity profile by using"p-sensitive readout optics" to image or project (hereinaftercollectively referred to as imaging) the light beam onto the recordingmedium

SUMMARY OF THE INVENTION

It has been found that proximity coupled electro-optic devices aresensitive to pressure differentials which are likely to exist if theelectrodes bear directly against the electro-optic element. If theelectro-optic element is a crystal, differential contact pressures tendto cause undesireable coupling between the electro-optic andpiezoelectric characteristics of the crystal, thereby detracting fromthe spatial uniformity of its electro-optic output. It has also beenfound that such devices are sensitive to variations in the physicaldisplacement of different points on the electrodes from theelectro-optic element, such as may be caused by the ordinary roughnessof the mating surfaces of the electrodes and the electro-optic element,by defects in those surfaces, and by dust or other particles which maybe entrapped therebetween.

Therefore, in accordance with this invention, the electrodes of aproximity coupled electro-optic device, such as a multi-gate light valvefor an electro-optic line printer, are mechanically gapped from theelectro-optic element to provide a relatively uniform gap distancebetween the electrodes and the electro-optic element, despite anyordinary surface roughnesses, entrapped dust particles, and evenconventional surface defects. To create the gap, there suitably arespaced apart rails disposed between the electro-optic element and theelectrode bearing substrate to maintain the electrodes at apredetermined small nominal gap distance from the electro-optic element.Preferably, the rails are located on opposite sides of the interactionregion of the electro-optic element so that no differential contactpressures are applied to the interaction region. The rails may be formedon the electrode support substrate to bear against the electro-opticelement or vice-versa.

BRIEF DESCRIPTION OF THE DRAWINGS

Still other features and advantages of this invention will becomeapparent when the following detailed description is read in conjunctionwith the attached drawings, in which:

FIG. 1 is a schematic side view of an electro-optic line printerincluding a proximity coupled TIR multi-gate light valve which embodiesthe present invention;

FIG. 2 is a schematic bottom plan view of the electro-optic line printershown in FIG. 1;

FIG. 3 is an enlarged side view of a TIR light valve for theelectro-optic line printer of FIGS. 1 and 2;

FIG. 4 is an enlarged cutaway bottom view of the TIR light valve of FIG.3 showing a pattern of individually addressable electrodes and the railswhich are provided to mechanically gap the electrodes from theelectro-optic element in accordance with this invention;

FIG. 5 is a simplified block diagram of a system for applyingdifferentially encoded serial input data to the individually addressableelectrodes of the electrode pattern shown in FIG. 4;

FIG. 6 is an enlarged and fragmentary sectional view of the TIR lightvalve shown in FIG. 3 to better illustrate the proximity coupling of theelectrodes to the electro-optic element and the interaction which occursbetween the light beam and the electric fringe fields within theinteraction region of the electro-optic element; and

FIG. 7 is an enlarged and fragmentary schematic plan view of theelectrode pattern of FIG. 4 as embodied on a silicon integrated circuitin accordance with this invention;

FIG. 8 is a enlarged cutaway bottom view of a TIR light valve having analternative electrode pattern in combination with the mechanical gappingrails provided in accordance with this invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

While the invention is described in some detail hereinbelow withreference to certain illustrated embodiments, it is to be understoodthat there is no intent to limit it to those embodiments. On thecontrary, the aim is to cover all modifications, alternatives andequivalents falling within the spirit and scope of the invention asdefined by the appended claims.

Turning now to the drawings, and at this point especially to FIGS. 1 and2, there is an electro-optic line printer 11 comprising a multi-gatelight valve 12 for exposing a photosensitive recording medium 13 in animage configuration. The recording medium 13 is depicted as being aphotoconductively coated xerographic drum 14 which is rotated (by meansnot shown) in the direction of the arrow. It nevertheless will beevident that there are other xerographic and non-xerographic recordingmedia that could be used, including photoconductively coated xerographicbelts and plates, as well as photosensitive film and coated paper in webor cut sheet stock form. The recording medium 13 should, therefore, bevisualized in the generalized case as being a photosensitive mediumwhich is exposed in an image configuration while advancing in a crossline or line pitch direction relative to the light valve 12.

As shown in FIGS. 3 and 4, the light valve 12 includes an electro-opticelement 17 and a plurality of individually addressable electrodes18a-18i. For a total internal reflection (TIR) mode of operation asillustrated, the electro-optic element 17 typically is a y cut crystalof, say, LiNbO₃ having an optically polished reflecting surface 21 whichis integral with and disposed between optically polished input andoutput faces 22 and 23, respectively. The electrodes 18a-18i areintimately coupled to the electro-optic element 17 adjacent thereflecting surface 21 and are distributed across essentially the fullwidth thereof. Typically, the electrodes 18a-18i are 1-30 microns wideand are on centers which are more or less equidistantly separated toprovide a generally uniform interelectrode gap spacing of 1-30 microns.In this particular embodiment the electrodes 18a-18i extend generallyparallel to the optical axis of the electro-optic element 17 and haveprojections of substantial length along that axis. Alternatively, theelectrodes 18a-18could be aligned at the so-called Bragg angle relativeto the optical axis of the electro-optic element 17. As will beappreciated, if the electrodes 18a-18i are aligned parallel to theoptical axis of the electro-optic element 17, the light valve 12 willproduce a diffraction pattern which is symmetrical about the zero orderdiffraction component. If, on the other hand, the electrodes 18a-18i areat the Bragg angle relative to the optical axis of the electro-opticelement 17, the light valve 12 will produce an asymmetrical diffractionpattern.

Briefly reviewing the operation of the line printer depicted in FIGS.1-4, a sheet-like collimated beam of light 24 from a suitable source,such as a laser (not shown), is transmitted through the input face 22 ofthe electro-optic element 17 at a grazing angle of incidence relative tothe reflecting surface 21. The light beam 24 is brought to a wedgeshaped focus (by means not shown) at approximately the centerline of thereflecting surface 21 and is totally internally reflected therefrom forsubsequent transmission through the output face 23. As will be seen, thelight beam 24 illuminates substantially the full width of theelectro-optic element 17 and is phase front modulated while passingtherethrough in accordance with the differentially encoded data samplesapplied to the electrodes 18a-18i.

More particularly, as shown in FIG. 5, serial input data samples, whichrepresent picture elements for successive lines of an image, are appliedto a differential encoder 25 at a predetermined data rate. The encoder25 differentially encodes the input samples on a line-by-line basis inresponse to control signals from a controller 26, and a multiplexer 27ripples the encoded data samples onto the electrodes 18a-18i at a ripplerate which is matched to the data rate in response to further controlsignals from the controller 26. The input data may, of course, bebuffered (by means not shown) to match the input data rate to anydesired ripple rate. Additionally, the input data may be processed (bymeans also not shown) upstream of the encoder 25 for text editing,formatting or other purposes, provided that the data samples for theultimate image are applied to the encoder 25 in adjacent picture elementsequence. See, for example, a commonly assigned U.S. patent applicationof William Gunning et al., which was filed Apr. 5, 1978 under Ser. No.893,658.

Differential encoding is described in substantial detail in a copendingand commonly assigned U.S. patent application of W. D. Turner et al.,which was filed Sept. 17, 1980 under Ser. No. 187,916 on "DifferentialEncoding for Fringe Field Responsive Electro-Optic Line Printers." Thusit will suffice to note that each differentially encoded data sample,other than the first sample for each line of the image, has a magnitudewhose difference from the previous differentially encoded data samplecorresponds to the magnitude of a respective input data sample. Thefirst sample for each line of the image is referenced to a commonreference potential, such as ground. Thus, all picture elements arefaithfully represented by the electrode to electrode voltage drops thatare produced in response to the differentially encoded data.

Referring to FIG. 6, the electrode to electrode voltage drops createlocalized fringe fields 28 within an interaction region 29 of theelectro-optic element 17, and the fringe fields 28 cause localizedvariations in the refractive index of the electro-optic element 17widthwise of the interaction region 29. The voltage drop between anyadjacent pair of electrodes, such as 18b and 18c or 18c and 18d,determines the refractive index for the portion of the interactionregion 29 which bridges between those two electrodes. Hence, therefractive index variations within the interaction region 29 faithfullyrepresent the input data samples appearing on the electrodes 18a-18i indifferentially encoded form at any given point in time. It thereforefollows that the phase front of the light beam 24 (FIG. 3) issequentially spatially modulated in accordance with the data samples forsuccessive lines of the image as the light beam 24 passes through theinteraction region 27 of the electro-optic element 17.

Returning for a moment to FIGS. 1 and 2, to expose the recording medium13 in an image configuration, there suitably are Schlieren central darkfield imaging optics 31 which are optically aligned between theelectro-optic element 17 and the recording medium 13 for imaging thelight beam 24 onto the recording medium 13. The imaging optics 31convert the spatial phase front modulation of the light beam 24 into acorrespondingly modulated intensity profile and provide anymagnification that is required to obtain an image of a desired width. Toaccomplish that, the illustrated imaging optics 13 include a field lens34 for focusing the zero order diffraction components 32 of the phasefront modulated light beam 24 onto a central stop 35 and an imaging lens36 for imaging the higher order diffraction components onto therecording medium 13, i.e., the image plane for the light valve 12. Thefield lens 34 is optically aligned between the electro-optic element 17and the stop 35 so that substantially all of the zero order components32 of the light beam 24 are blocked by the stop 35. The higher orderdiffraction components of the light beam 24 scatter around the stop 35and are collected by the imaging lens 36 which, in turn, causes them tofall onto the light valve image plane defined by the recording medium13. Of course, other p-sensitive readout optics could be used to convertthe phase front or polarization modulated light beam provided by theelectro-optic element 17 into a light beam having a correspondinglymodulated intensity profile.

To summarize, as indicated in FIG. 2 by the broken lines 39, eachneighboring pair of electrodes, such as 18b and 18c (FIG. 6), cooperateswith the electro-optic element 17 and with the p-sensitive readoutoptics 31 to effectively define a local modulator for creating a pictureelement at a unique, spatially predetermined position along each line ofthe image. Accordingly, the number of electrodes 18a-18i determines thenumber of picture elements that can be printed per line of the image.Moreover, by sequentially applying successive sets of differentiallyencoded data samples to the electrodes 18a-18i while the recordingmedium 13 is advancing in a cross line direction relative to the lightvalve 12, successive lines of the image are printed.

As best shown in FIG. 7, the electrodes 18a-18i are preferably definedby a suitably patterned, electrically conductive layer, generallyindicated by 30, which is deposited on and is a part of an electricalintegrated circuit 31, such as a LSI (large scale integrated) siliconintegrated circuit, to make electrical contact to the integrated driveelectronics 32b-32g. For example, as illustrated, the multiplexer 27 isembodied in the integrated circuit 31, and the electrodes 18a-18i are anextension of the metalization or polysilicon layer 30 which is used tomake electrical connections to the output transfer gates or passtransistors 32b-32g and other individual components (not shown) of themultiplexer 27. The pass transistors 32b-32g and the other components ofthe multiplexer 27 are formed on the integrated circuit 31 by using moreor less standard LSI component fabrication techniques, and themetalization or polysilicon layer 30 is thereafter deposited on theouter surface 33 of the integrated circuit 31. An etching process or thelike is then used to pattern the electrically conductive layer 30 asrequired to provide electrically independent connections to theelectrically independent components of the multiplexer 27 and to formthe electrically independent electrodes 18a-18i(only the electrodes18b-18g can be seen in FIG. 7). Thus, the data transfer lines 34b-34gfor the pass transistors 32b-32g are defined in the metalization orpolysilicon layer 30 by the same etching process which is used to definethe electrodes 18a-18i.

Referring again to FIG. 6, the electrodes 18a-18i are proximity coupledto the electro-optic element 17. To that end, a clamp, which isschematically represented by the arrows 42 and 43, is engaged with theelectro-optic element 17 and with the silicon integrated circuit 31 tourge the electrodes 18a-18i into close contact with the reflectingsurface 23 of the electro-optic element 17. Alternatively, theintegrated circuit 31 could be bonded to the electro-optic element 17 byan adhesive or by suction.

In accordance with the present invention, as shown in FIGS. 4 and 6,rails 44 and 45 or similar spacers are provided to mechanically maintainthe electrodes 18a-18i at a small predetermined nominal gap distancefrom the electro-optic element 17. The rails 44 and 45 are preferablylocated on opposite sides of the interaction region 27 of theelectro-optic element 17 so that the interaction region 27 is free ofpressure differentials which might cause undesireable piezoelectriceffects. For example, as shown the rails 44 and 45 are formed on theelectrode bearing integrated circuit 31 near its lead edge and trailedges, repectively, to engage the electro-optic element 17 forward andaft, respectively, of the interaction region 27. Alternatively, therails 44 and 45 could be formed on the integrated circuit 31 near itsfront and back edges, repectively, to engage the electro-optic element17 on opposite lateral sides of the interaction region 27. Still anotheralternative is to form the rails 44 and 45 on the electro-optic element17 to engage the electrode substrate 31 either forward and aft or onopposite lateral sides of the electrodes 18a-18i. The interelectrode gapspacing of the electrodes 18a-18i must be sufficiently large compared tothe nominal gap spacing provided by the rails 44 and 45 to ensure thatthe fringe fields 28 span the gap 46 and penetrate the electro-opticelement 17 as previously described. For example, the rails 44 and 45 areeach selected to have sufficient to provide a nominal gap 46 on theorder of 0.10 microns if the interelectrode gap spacing is on the orderof 5 microns. Of course, increased or decreased interelectrode gapspacings may require or permit a corresponding change in the heights ofthe rails 44 and 45.

A hard dielectric film 47 may be overcoated on the electrodes 18a-18i tophysically protect the electrodes 18a-18i and to isolate any pointdefects in the silicon integrated circuit 31 from the gap 46. The film47 preferably has a dielectric constant to thickness ratio which ishigher than the corresponding ratio for an air gap of the samethickness, so that it does not appreciably increase the effectiveelectrical thickness of the gap 46 as measured by the voltage dropoccurring between any one of the electrodes 18a-18i and theelectro-optic element 17.

As shown in FIG. 8, the present invention may also be used tomechanically gap an alternating pattern of individually addressableelectrodes 18a¹ -18i¹ and ground plane electrodes 19a¹ -19i¹ from theelectro-optic element 17. As is known, such an electrode pattern may beused if the input data samples are not differentially encoded.

CONCLUSION

In view of the foregoing, it will be understood the mechanical gappingprovided by this invention eliminates several possible sources ofunwanted noise in the output of a proximity coupled electro-opticdevice, such as multi-gate light valve for an electro-optic lineprinter.

What is claimed is:
 1. In an electro-optic device includinganelectro-optic element, a plurality of electrodes intimately coupled tosaid electro-optic element, and means coupled to said electrodes forapplying voltages thereto, whereby electric fields are created withinsaid electro-optic element; the improvement comprising a substrate, atleast one of said electrodes being supported by said substrate, meansengaged with said electro-optic element and with said substrate forbonding said substrate to said electro-optic element, and spacer meansdisposed between said substrate and said electro-optic element forgapping said substrate supported electrodes from said electro-opticelement, thereby causing said fields to proximity couple into saidelectro-optic element.
 2. The improvement of claim 1 whereinsaid spacermeans maintain a predetermined nominal gap distance between saidsubstrate supported electrodes and said electro-optic element.
 3. Theimprovement of claim 2 whereinsaid spacer means are disposed on oppositesides of said substrate supported electrodes, whereby said electricfields are coupled into a region of said electro-optic element which issubstantially free of differential contact pressures.
 4. The improvementof claim 3 whereinsaid substrate is a integrated electrical circuitcontaining integrated electronics for applying said voltages to saidsubstrate supported electrodes, and further including a conductive layerdeposited on said integrated circuit for making electrical connectionsto said electronics, said conductive layer being patterned to definesaid substrate supported electrodes.
 5. The improvement of claim 4whereinsaid integrated circuit is a silicon integrated circuit, and saidspacer means are integral with said silicon integrated circuit, and saidbonding means maintains said spacer means in contact with saidelectro-optic element to provide said nominal gap spacing.
 6. Theimprovement of claim 2 whereinsaid electro-optic element is opticallytransmissive and has a predetermined optical axis and a reflectivesurface which is generally parallel to said optical axis, saidelectro-optic device further includes means for applying a sheet-like,collimated light beam to said electro-optic element at a grazing angleof incidence relative to said reflective surface, whereby said lightbeam is totally internally reflected from said reflective surface, andall of said electrodes are supported on said subtrate in spaced apartrelationship orthogonally to said optical axis, and said electrodes haveprojections of substantial length along said optical axis and aninterelectrode gap spacing which is large relative to said nominal gapdistance, whereby said electrodes create fringe fields within aninteraction region of said electro-optic element in response to saidvoltages to modulate said light beam in accordance with said voltageswhile said light beam approaches toward and recedes from said reflectivesurface.
 7. The improvement of claim 6 whereinsaid spacer means aredisposed on opposite sides of said electrodes, said substrate is anintegrated circuit containing electronics for applying said voltages tosaid electrodes, and further including a conductive layer deposited onsaid integrated circuit for making electrical connections to saidelectronics, said conductive layer being patterned to define saidelectrodes.
 8. The improvement of claim 7 whereinsaid integrated circuitis a silicon integrated circuit, and said spacer means are integral withsaid silicon integrated circuit and are maintained in contact with saidelectro-optic element by said bonding means.
 9. The improvement of claim2 whereinsaid electro-optic element is optically transmissive and has apredetermined optical axis, said electro-optic device further includesmeans for transmitting a sheet-like, collimated light beam through saidelectro-optic element in an axial direction, all of said electrodes aresupported on said substrate whereby electric fringe fields are createdin said electro-optic element in response to said voltages, saidelectrodes being spaced apart orthogonally relative to said optical axisand widthwise of said light beam, at least every other one of saidelectrodes is independently addressable, and said means for applyingvoltages to said electrodes includes means for cyclically applyingrespective data samples to said independently addressable electrodes,the data samples applied during any one of said cycles representingpicture elements for a respective line of an image, and the data samplesapplied during successive cycles representing picture elements forsuccessive lines of said image, whereby said fringe fields sequentiallyspatially modulate said light beam in accordance with the pictureelements for successive lines of said image.
 10. The improvement ofclaim 9 whereinsaid spacer means are disposed on opposite sides of saidelectrodes, whereby said fringe fields are coupled into an interactionregion of said electro-optic element which is substantially free ofdifferential contact pressures.
 11. The improvement of claim 10whereinsaid substrate is an integrated electrical circuit containingelectronics for applying said data samples to said electrodes, andfurther including a conductive layer deposited on said integratedcircuit for making electrical connections to said electronics, saidconductive layer being patterned to define said electrodes.
 12. Theimprovement of claim 11 whereinsaid integrated circuit is a siliconintegrated circuit, said spacer means are rail-like members extendingfrom said integrated circuit on opposite sides of said electrodes, andsaid bonding means maintains said rail-like members in contact with saidelectro-optic element on opposite sides of said interaction region,whereby said interaction region is substantially free of anydifferential contact pressures.
 13. The improvement of claim 11whereinsaid electro-optic element is a crystal having a reflectivesurface which is generally parallel to said optical axis said light beamis applied to said crystal at a grazing angle of incidence relative tosaid reflective surface, whereby said light beam is totally internallyreflected from said reflective surface, and said bonding means maintainssaid rail-like members in contact with said reflective surface toproximity couple said electrodes to said electro-optic element, and saidelectrodes have projections of substantial length along said opticalaxis and an interelectrode gap spacing which is larger than said nominalgap distance, whereby said electrodes create fringe fields within theinteraction region of said crystal in response to said data samples tomodulate said light beam in accordance with said data samples while saidlight beam is approaching toward and receding from said reflectivesurface.
 14. The improvement of claim 12 whereinsaid integrated circuitis a silicon integrated circuit, said spacer means are rail-like membersextending from said integrated circuit on opposite sides of saidelectrodes, and said bonding means maintains said rail-like members incontact with said crystal on opposite sides of said interaction region,whereby said interaction region is substantially free of anydifferential contact pressures.